This invention relates to a method of providing, in a dual-worm extruder, a wear-resistant layer on the inner surface of the housing bore which accommodates two parallel-extending extruder worms and has, accordingly, an approximate cross-sectional shape of a figure "8".
Extruder or injection machines are widely used in many fields for making bodies of plastics or rubber. Extruders are known which have only a single worm or which have two axially parallel, cooperating worms for advancing the material to be injected. A dual worm has, as compared to a single worm, the advantage that the material is discharged in a more uniform manner. The housing for such a dual worm is so structured that the bore has, in cross section, the shape of an open figure "8", that is, both housing parts for the worms are open at the parting plane. There are known dual worm extruders which are provided with cylindrical housing bores and extruders are further known in which the bores (and thus the worms as well) have a conical shape. Such a conical configuration is of advantage, since the pressure required for discharging the material is easier to generate.
Since the worms rotate in the extruder housing and since the clearance between the worms and the housing has to be very small to ensure that the required pressure can be built up, the inner surface of the housing bore is exposed to a continuous wear. Consequently, this surface has to be particularly wear resistant. Heretofore, as a rule, this surface has been either hardened by a special heat treatment or has been provided with a thin layer of wear-resistant material applied by a centrifugal coating process at relatively high temperatures. Both processes, however, involve very significant expense and require substantial subsequent finishing work. Particularly in case of centrifugal coating often shrinkages occur on the surface which cannot be remedied by a subsequent treatment so that the entire extruder housing has to be eliminated as waste.